Pulmonary embolism (PE) is a common post-surgical complication for which management is suboptimal. Use of plasminogen activators (PA) in the post-operative period is restricted to patients with submassive or massive PE otherwise eligible for embolectomy because of its lack of proven clinical effectiveness and risk of hemorrhage. There is a pressing need to better understand why lysis of PE fails to generate better outcomes and new means to circumvent this problem. Our recent studies help to explain what we term the pulmonary fibrinolytic paradox. Our findings implicate activation of lipoprotein receptor-related protein receptor (LRP1) and N-methyl-D-aspartate (NMDA) receptors (NMDARs) in the pulmonary vasculature by uPA (and tPA) which disrupts pulmonary arterial EC barrier function and deregulates vascular tone, offsetting the salutary benefits of fibrinolysis. We hypothesize that uPA signals through LRP1 to induce vasoconstriction in partially occluded hypoxic vessels, while activating NMDARs in well-perfused segments, which generates an overriding signal that leads to vasodilation. This diverts blood from underperfused ischemic segments and exacerbates ventilation/perfusion mismatch. Fortunately, we have successfully segregated the deleterious non-fibrinolytic vasoactive effects from the salutary fibrinolytic activity of uPA, sho each is mediated by distinct domains that can be blocked by specific peptides and receptor antagonists and bypassed with specific uPA mutants. We will elucidate the receptor-mediated pathways activated by uPA that disrupt pulmonary endothelial barrier function and increase vascular tone and use these insights to develop novel fibrinolytics and delivery systems with a greater benefit to risk ratio for use to prevent and manage PE in the perioperative period through three interrelated Aims: In Aim 1, we will delineate the mechanism of uPA-induced transendothelial self-transport and pulmonary vasoconstriction by assessing of contributions of LRP and NMDAR under hypoxic vs normoxic conditions in vitro and in vivo and assess the efficacy of novel vasoneutral uPA variants. In Aim 2, we will assess the relationship between vasoactivity, fibrinolysis and clinical outcome in a model of sub-massive PE and the proposed enhanced effectiveness of vasoneutral uPA variants. In Aim 3, we will explore the thromboprophylactic potential of RBC-targeted vasoneutral uPAs that form intrafibrin channels that permit rapid transport of erythrocytes to salvage ischemic tissues prior to complete clot lysi without affecting post-operative hemostatic clots. Thus, our studies are of direct translational relevance because they: 1) identify a novel class of receptors, agonists, receptors and inhibitors that impair the clinical efficacy of PAs, 2) a process that can be bypassed by re-engineering uPA, and 3) describe a new approach to safe and effective thromboprophylaxis that we hope will improve perioperative prevention and treatment of PE.